Quasi-elliptic function microstrip interdigital filter

Abstract

The Frequency response of a Microstrip Interdigital Filter (MIDF) is improved by providing a specially dimensioned top cover separated from the MIDF ground plane along one or both of its edges by a dielectric layer comprised of thin dielectric sheets. While the upper frequency skirt is mainly determined by the MIDF itself, the bandwidth may be reduced and the slope of the lower frequency skirt may be enhanced by providing the cover in accordance with the invention. The thickness of the dielectric layer can be chosen to further improve the slope of the low frequency skirt by effectively moving the filter zero closer to the desired low frequency cut-off of the filter. The particular choice of the dielectric thickness will not substantially affect the filter bandwidth.

I claim: 
     
       1. In a microstrip interdigital filter having a desired passband, said passband comprising at least a lower frequency skirt, having a desired location and slope, said microstrip interdigital filter having a supporting structure and a microwave circuit mounted thereon; means for effecting a change in (1) said location of said low frequency skirt and (2) said slope of said low frequency skirt comprising: (a) cover means having first and second ends fixedly attached to said supporting structure so as to cover said filter;   (b) a first dielectric layer located between said supporting structure and said first end of said cover means.   
     
     
       2. The filter of claim 1 wherein said location of said lower frequency skirt is substantially determined by the dimensions of said cover means, and said slope of said lower frequency skirt is substantially determined by the thickness of said dielectric layer. 
     
     
       3. The filter of claim 2 further comprising a second dielectric layer located between said supporting structure and said second end of said cover means, whereby said slope of said lower frequency skirt is further determined by the presence of said second dielectric layer. 
     
     
       4. The filter of claims 1, 2, or 3 wherein said microwave circuit includes a plurality of substantially parallel interdigital portions mounted on a dielectric substrate deposited on said supporting structure; wherein said cover means covers said interdigital portions and is (i) open in a direction substantially perpendicular to said parallel interdigital portions and (ii) affixed to said supporting structure at ends running parallel to said interdigital portions. 
     
     
       5. The filter of claim 4 wherein said cover means is electrically conductive. 
     
     
       6. The filter of claim 5 wherein said cover means is attached to said supporting structure at said first end by dielectric connector means, whereby said supporting structure is conductively isolated from said cover means at said first end. 
     
     
       7. The filter of claim 3 wherein said microwave circuit includes a plurality of substantially parallel interdigital portions mounted on a dielectric substrate deposited on a supporting structure; said cover means is electrically conductive, covers said interdigital portions and is (i) open in a direction substantially perpendicular to said parallel interdigital portions, and (ii) affixed to said supporting structure at said first and second ends by dielectric connector means, whereby said supporting structure is conductively isolated from said cover means, said first and second ends running parallel to said interdigital portions. 
     
     
       8. The filter of claim 6 wherein said first dielectric layer is comprised of polyethylene. 
     
     
       9. The filter of claim 7 wherein first and second dielectric layers are comprised of polyethylene.